The tight junction (TJ) is the apical-most intercellular junction complex, serving as a biological barrier of intercellular spaces between epithelial cells. The TJ’s integrity is thought to be maintained by a key protein–protein interaction between C-terminal motifs of claudins (CLDs) and the postsynaptic density 95 (PSD-95)/discs large/zonula occludens 1 (ZO-1; PDZ) domains of ZO-1. Weak but direct interaction of baicalin, baicalein, and hesperetin—which are known pharmacologically active flavonoids—with ZO-1(PDZ1) have been assessed by solution NMR experiments. We observed TJ-mitigating activity of these flavonoids against Madin-Darby canine kidney (MDCK) II cells with the downregulation of subcellular localization of CLD-2 at TJs. In addition, we also found that quercetin, another physiologically relevant flavonoid, showed down regulation of TJ integrity, although quercetin did not bind ZO-1(PDZ1). We propose that inhibition of protein-protein interaction mediated by PDZ domains of TJ scaffolding proteins is one of the potential mechanisms for TJ-mitigation. We further confirmed that 1.2 times increase of paracellular efflux of FITC-labeled insulin of baicalin and baicalein on Caco-2 cell monolayers. In addition, a marked decrease of TEER by quercetin on Caco-2 cell monolayers was observed. These results suggested some of flavonoids are useful as a drug absorption enhancer.
Support or Funding Information
This work was supported in part by the Japan Science and Technology Agency (JST), the Target Protein Research Program, Japan Science and Technology Agency, A-step Feasibility Study Program (AS262Z01275Q, AS242Z00566Q), Japan Society for the Promotion of Science KAKENHI (15H04337) and KOSÉ Cosmetology Research Foundation.
Tracer efflux experiment of FITC-labeled insulin using Caco-2 cell monolayers. The cells were treated with 300 μM of baicalin and baicalein for 48 hours.
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